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Molecular and Cellular Biology, August 2007, p. 5887-5897, Vol. 27, No. 16
0270-7306/07/$08.00+0     doi:10.1128/MCB.00175-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Zebra fish cassiopeia Mutant Reveals that SIL Is Required for Mitotic Spindle Organization ^,

Kathleen L. Pfaff, Christian T. Straub, Ken Chiang, Daniel M. Bear, Yi Zhou, and Leonard I. Zon^*

Stem Cell Program and Division of Hematology/Oncology, Children's Hospital Boston, Dana Farber Cancer Research Institute, Howard Hughes Medical Institute, and Harvard Medical School, Karp Family Research Building, 1 Blackfan Circle, Boston, Massachusetts 02115

Received 30 January 2007/ Returned for modification 9 March 2007/ Accepted 6 June 2007

A critical step in cell division is formation of the mitotic spindle, which is a bipolar array of microtubules that mediates chromosome separation. Here, we report that the SCL-interrupting locus (SIL), a vertebrate-specific cytosolic protein, is necessary for proper mitotic spindle organization in zebrafish and human cells. A homozygous lethal zebrafish mutant, cassiopeia (csp), was identified by a genetic screen for mitotic mutant. csp mutant embryos have an increased mitotic index, have highly disorganized mitotic spindles, and often lack one or both centrosomes. These phenotypes are caused by a loss-of-function mutation in zebrafish sil. To determine if the requirement for SIL in mitotic spindle organization is conserved in mammals, we generated an antibody against human SIL, which revealed that SIL localizes to the poles of the mitotic spindle during metaphase. Furthermore, short hairpin RNA knockdown of SIL in human cells recapitulates the zebrafish csp mitotic spindle defects. These data, taken together, identify SIL as a novel, vertebrate-specific regulator of mitotic spindle assembly.

Published ahead of print on 18 June 2007.

Supplemental material for this article may be found at http://mcb.asm.org/.

Present address: Division of Developmental Biology, Children's Memorial Hospital/Northwestern University, Chicago, IL 60640.

Present address: Pritzker School of Medicine, The University of Chicago, Chicago, IL 60637.